There is known an art of passing a common cooling medium through an internal combustion engine and a stirling engine. Patent Document 1 discloses a power unit equipped with a water-cooled internal combustion engine having a cooling circuit, and a stirling engine having a cooling device that cools a working gas by a coolant that circulates through the cooling circuit. Further, patent Documents 2 through 4 disclose arts that are considered as being relative to the present invention in terms of structure.